1. Field of the Invention
This invention relates to logic circuits and in particular to a logical element known as a PASS transistor and to logic circuits formed using PASS transistors so as to substantially reduce the number of active devices required to implement logic functions.
2. Prior Art
Boolean logic expressions may be realized through classical logic design methods with networks of combinational logic. Logic networks using MOS technology are often formed with NAND, NOR and Inversion gates since these structures are easily implemented. The PASS transistor is another common MOS structure. Classical logic design methods, however, do not result in networks of PASS transistors.
FIG. 1a shows the PASS transistor with its logical function. When the gate or control electrode 13 to PASS transistor 10 is asserted (and by "asserted" is meant driven to a voltage so as to form a conductive path between input lead 11 and output lead 12), transistor 10 passes the logic state at input lead 11 to output lead 12. The input and output leads of an MOS transistor may be arbitrarily chosen because an MOS transistor is bidirectional. For convenience, the input lead shall hereafter be defined as the terminal driven by the source of a logic state. The output lead 12 of PASS transistor 10 typically applies the passed logic state to the input lead of another series connected logic structure. Output leads of a number of PASS transistors may be joined to drive the same input of a logic structure provided that all PASS transistors whose controls are asserted are passing the same logic state. The use of PASS transistors as bidirectional transmission gates is described in an article entitled "VLSI: A New Frontier for Systems Designers" published in the January 1982 Computer Journal of the IEEE by Douglas G. Fairbairn, which is hereby incorporated by reference.